Just to be clear, I'm having trouble with just one skin; there are over two-dozen other custom player skins and models in my base folder and they all appear to work fine (I even checked the standard models to be sure).
The skin I'm trying to load is based on the Shadowtrooper, JA version. The first time I tried to load it, I used "/model [skin's name]," but ended up loading Kyle's model AND soundset instead (taunt is "Just taking care of business" instead of the default Reborn laugh). I thought it might've been an issue with MV's support for JA player models so I replaced the model.glm with the original JO version (and modified sounds.cfg appropriately), but the symptom persisted. I then installed and tried the skin in my Steam copy of JO and found that it works perfectly.
I've taken several looks at the content of the pk3 and everything appears in order. The .skin files have the correct paths, no spelling errors to be found, and the icons work in the player menu, but this one skin just doesn't want to work in MV and I don't know what else I can do to try and solve it.